1. Field of the Invention
The present invention relates to electro-optical device such as display devices used for computers, word processors, etc., and those which display TV video signals sent from ground TV stations, satellite TV stations, cable TV stations, etc., or video signals delivered from picture recording systems (e.g., video deck, laser disc player, magneto-optical disc system, etc.). The electro-optical device of the present invention can also be used for the view-finder of a video camera.
2. Description of the Prior Art
Hitherto, the STN (Super Twist Nematic) type liquid crystal display has commonly been used for the display screen of a computer, a word processor, etc. Since the liquid crystal material of the STN type display has steep electro-optical characteristics in comparison to the TN (Twist Nematic) type liquid crystal display used formerly, it enables high-level time-division drive for a large amount of information, which is difficult to realize with the TN type liquid crystal display. Thus, the advent of the STN type liquid crystal display triggered the development of so-called "note" (i.e., portable) personal computers and word processors, which are now widespread. However, as the number of scanning lines is increased to perform the time division drive, it becomes difficult to ensure the required ratio of voltages applied to selected and non-selected scanning lines, respectively, resulting in a lowering in the display qualities, particularly contrast.
A typical conventional device that displays images processed from video signals sent from a ground TV station, a satellite TV station, a cable TV station, or a TV picture recording system (video deck, laser disc player, magneto-optical disc system, etc.) employs a method in which a fluorescent screen that forms a display screen is irradiated with electron beams in a vacuum tube, called Braun tube or CRT (Cathode-Ray Tube), thus causing the fluorescent screen to emit visible light.
At the beginning, display devices with screens having a diagonal dimension of 12 to 14 inches were in common use. Recently, however, large display devices with a screen size of 20 to 30 inches or more have appeared by the request of society.
When the diagonal dimension of a display screen is 30 inches, the depth is also about 30 inches, and the thickness of glass that forms the screen exceeds 1 centimeter in order to ensure the required strength.
Further, a system in which an image on a cathode-ray tube of high brightness is enlarged through an optical system and projected onto a screen has been proposed and already used for devices that have a large display area. The arrangement of this system is shown schematically in FIG. 1.
In the case of television receivers that use a cathode-ray tube, if the size of the display screen exceeds 30 inches, the overall weight is well over 100 kg. In an ordinary home, it is not easy to find a place to put a heavy object of more than 100 kg in weight. With such a heavy weight, the television receiver once installed is difficult to move by human power on such an occasion that the layout needs to be changed, which is an obstacle to the spread of wide-screen television receivers to ordinary homes.
To solve the problem of weight, projection television receivers have been proposed. However, the brightness per se of the enlarged screen is considerably low because of the limitation on the improvement in the brightness of the high-brightness cathode-ray tube, on which this type of television system is based. For this reason, the screen is rather dark, and since the screen is enlarged through an optical system, although the contrast ratio of the screen as seen from the front is high, that of the screen as seen from an oblique direction is exceedingly low in comparison to the cathode-ray tube type television receiver. However, the projection television receiver solves the problem of weight since its weight is about 50% of that of the cathode-ray tube type. The projection television 201 shown in FIG. 1 comprises a cathode-ray tube or liquid crystal display 204, a tuner 205, an optical system 203, a reflector 202, and a screen 206.
Recently, display devices that employ a thin-film transistor liquid crystal display using amorphous silicon as a display member in place of a cathode-ray tube have been proposed. FIG. 2 shows the structure of one of the proposed display devices. The relatively light weight of this type of display device, which is only about 30% of that of the cathode-ray tube type, has helped the spread of these devices to ordinary homes. However, the brightness of the display screen of this type of display device is low in comparison to the cathode-ray tube type. Although efforts have been made to solve the problem of low brightness by increasing the light intensity of the light source, if the light source intensity is increased, there is a lowering in the resistance of the thin-film transistors when in an "off" state due to a rise in temperature of the liquid crystal panel and the illumination with light, so that it is difficult to attain satisfactory display. FIG. 3 shows a typical conventional active matrix arrangement. For simplification of illustration, the arrangement is shown in the form a matrix of 2 rows and 2 columns. A plurality of gate lines G1 and G2 and a plurality of data lines D1 and D2 are disposed to intersect each other perpendicularly, and pixel display elements are provided at the intersections, respectively. Each pixel display element comprises a TFT portion 501 and a liquid crystal portion 502. Signals from peripheral circuits 506 and 507 are applied to the pixels to selectively turn on or off given pixels.
However, in actual use of these liquid crystal display devices, it is often that the output of the TFT 501, that is, the input voltage V.sub.LC to the liquid crystal 502 (hereinafter referred to as "liquid crystal potential"), fails to become "1" (High) when it should do so, and also fails to become "0" when required to do so. This problem is attributable to the fact that a switching element that applies a signal to a pixel, that is, the TFT 501, lacks symmetry in the characteristics thereof. In other words, the way in which the pixel electrode is charged and the way in which it is discharged are not in symmetry with respect to each other. The liquid crystal 502 operates as an insulator, and the liquid crystal potential (V.sub.LC) is in a floating state when the TFT 501 is in an "off" state. In addition, since the liquid crystal 502 is equivalent to a capacitor, V.sub.LC is determined by the electric charge stored in it. When the resistance of the liquid crystal becomes relatively small at R.sub.LC, or when a leakage occurs owing to the presence of dust or ionic impurities, or when R.sub.GS occurs due to a pinhole in the gate insulator of the TFT 501, the stored charge leaks, so that V.sub.LC becomes ambiguous. For this reason, no high yield can be attained for a liquid crystal display having 200,000 to 5,000,000 pixels on a single panel.
In addition, it is necessary in order to effect color display to use at least three active matrix display devices as display members, as shown in FIG. 2. Accordingly, the low yield is raised to the third power at least, resulting in a considerably low overall yield, and thus inviting a rise in the cost.
There is another type of display that uses a ferroelectric liquid crystal material, which was proposed by Clark Lagawall et al. FIG. 4 is a conceptual view of this type of liquid crystal display. Since a ferroelectric liquid crystal material has spontaneous polarization, when the liquid crystal layer is thinned to such an extent that it is unspiraled, a surface stable ferroelectric liquid crystal (SSFLC) state is attained, so that it is possible to obtain the memory effect that once an electric field is applied thereto, a transparent or opaque state continues even if the electric field is removed. By making use of this memory state, it is possible to realize a static drive similar to that in the case of the active matrix LCD using TFTs.
However, since the ferroelectric liquid crystal can have only two stable states, that is, transparent and opaque states, it is not suited for tonal display (gradation display) that is required as a result of diversification of information. Tonal display is essential particularly when these liquid crystal electro-optical devices are used for video purposes. To solve this problem, it is conventional practice to divide each unit pixel into a multiplicity of areal sections so that it comprises a plurality of dots to effect tonal display. For example, a method has been invented in which each unit pixel is divided into three sections in the areal ratios of 1:2:4 and the "on" and "off" states of these divided sections are combined to obtain 8 gradation levels. FIG. 5 (a) shows the electrode structure designed for tonal display with 2 gradation levels , and 5 (b) shows that for tonal display with 8 gradation levels.
With the prior art method, however, three data signals must be applied per unit pixel, so that the external circuit configuration becomes extremely complicated, resulting in a rise in the cost and a lowering in the yield at the time of connection of the external circuits. In addition, since the division of the unit pixels needs insulating sections between the neighboring electrodes, the aperture ratio lowers. For example, if a display screen in which unit pixels are arranged at a pitch 250 .mu.m and with a gap of 25 .mu.m is considered, when the unit pixels are not divided, the aperture ratio is 81%, whereas, when they are divided with the same gap, the aperture ratio lowers to 63%. In addition, owing to the division, the width of the thinnest electrodes 103 is 25 .mu.m when the pitch and the gap are the same as the above. Thus, even when ITO that has a sheet resistance of not higher than 5 .OMEGA. is used to form a liquid crystal display of 1,000.times.1,000 pixels, electrodes that are disposed in the data line direction from one end to the other have a resistance of about 50 k.OMEGA.. Accordingly, there is an intensity difference between the electric fields that are applied to the liquid crystals at two ends of each electrode, so that no uniform display can be performed. Thus, the prior art lacks practicability.